Some prior art hydraulic actuators, such as the actuators disclosed in WO 96/07029 or U.S. Pat. No. 4,870,892 provide safety measures in case of electrical power cut off or actuator failure. In the actuators disclosed in WO 96/07029 and U.S. Pat. No. 4,870,892 this is obtained by causing a sliding member to move to a neutral position in the case of power cut off or actuator failure. In the actuator disclosed in U.S. Pat. No. 4,870,892 this is obtained by positioning the sliding member in the diagonal of a bridge circuit. Two valves which are normally closed in a de-energized state are fluidly connected between a pump and two pressure chambers of the sliding member, and two valves which are normally open in a de-energized state are fluidly connected between the pressure chambers and a tank. Furthermore, two biasing springs are arranged in the sliding member, biasing the sliding member towards the neutral position. Thus, in the case of an electrical power cut off the two valves arranged on the pump side are closed and the two valves on the tank side are opened. In the absence of fluid pressure from the pump, due to the closed valves on the pump side, the biasing springs will push the sliding member towards the neutral position, and fluid is allowed to flow between the tank and the pressure chambers, due to the open valves on the tank side. Accordingly, the sliding member is moved into the neutral position.
In the actuator disclosed in WO 96/07029 a sliding member is also positioned in the diagonal of a bridge circuit. However, in this case all four valves are of the normally open type. Thus, in the case of a power cut off, all four valves are opened. Thereby there is a permanent flow of fluid from the pressure source (pump) to the pressure sink (tank). Since this flow of fluid is distributed uniformly over the two branches of the bridge circuit, the pressure on each side of the sliding member is the same. Thereby the sliding member will be moved to a neutral position.
U.S. Pat. No. 4,416,187 discloses an actuator configuration based on switching valves where an instantaneous locking of a sliding member is possible. In the actuator disclosed in U.S. Pat. No. 4,416,187 a sliding member is positioned in the diagonal of a bridge circuit of valves. Two check valves are fluidly connected between a pump and two pressure chambers of the sliding member, the check valves being arranged in such a manner that a fluid flow is prevented in a direction from the pressure chambers towards the pump. Furthermore, two valves of the normally closed type are fluidly connected between the pressure chambers and a tank. Thus, in the case of a power cut off, the two valves of the normally closed type are closed. Thereby fluid is not allowed to flow out of the pressure chambers, and the sliding member is thereby locked in its instantaneous position.
However, in some hydraulic position motors, such as those incorporated in hydro-mechanical transmissions (HMT's) on all terrain vehicles or work utility vehicles, there is a need for hydraulically locking the sliding member in one of its end positions, if an actuator failure is detected or loss of power happens. For example, when a failure occurs in the actuation system or electrical power is lost in a HMT of a vehicle, an operator needs to drive the vehicle to a repair shop. In this particular situation it is desirable that the HMT shifts gently towards a specific gear ratio and is locked in this gear ratio. With regard to the hydraulic servomotor, this means that it should gently slide to a desired position, preferably an end position, and stay there.